1. Field of the Invention
The present invention relates to an image reading system capable of black-and-white and color image reading in two modes, and adapted for use in a facsimile or a scanner.
2. Related Background Art
For reading a color original, there is already known a color image sensor with light source switching, in which light emitting diodes (LED) are provided for emitting lights of three primary colors of R(red), G(green) and B(blue) and a signal is obtained from a photosensor by irradiating each position of the original with each of the R, G and B lights whereby a color signal is obtained corresponding to the color original.
The drive of such light source-switched color image sensor is controlled by an image sensor driving circuit 101 as shown in FIG. 35, wherein provided are a light source-switched color image sensor unit 200, a main controller 102 for controlling the drive thereof, a control signal generating circuit 103 for generating control signals XSH, MCLK in response to a control signal CNT from the main controller 102, an LED control unit 104 for generating signals, ΦR, ΦG, ΦB for controlling the turning-on of the R, G, B LEDs in response to the control signals CNT and XSH, and a sensor array control unit 105 for generating signals SP, CLK for controlling the drive of a sensor array in response to the control signal XSH, MCLK.
In such image sensor driving circuit, the main controller 102 sends the control signal CNT matching the reading mode to the control signal generation circuit 103 and the LED control circuit 104, thereby controlling the turning-on of the R, G and B LEDs and driving the sensor array according to the reading mode.
In the color original reading mode, the image sensor drive circuit 101 supplies the light source-switched image sensor unit 200 with the control signals ΦR, ΦG, ΦB, SP and CLK as shown in FIG. 36, to effect the image reading in the following manner.
At first the signal ΦR turns on the R LED only, and the sensor array is activated by the start pulse SP and the clock pulse CLK to accumulate R signals in the pixels of the sensor array. After the lapse of an R signal accumulation time tron12, the R LED are turned off by the signal ΦR. Then the G LED are turned on by the signal ΦG and the start pulse SP is entered again, whereupon the R signals already accumulated in the pixels of the sensor array are simultaneously transferred to analog memories on the sensor array and then are output to the exterior pixel by pixel.
After the same time, G signals are accumulated in the pixels of the sensor array. After the lapse of a G signal accumulation time tgon12, the G LED are turned off by the signal ΦG. Then B LED are turned on by the signal ΦB and the start pulse SP is entered again, whereby the G signals already accumulated in the pixels of the sensor array are simultaneously transferred to analog memories on the sensor array and then are output to the exterior pixel by pixel.
At the same time, B signals are accumulated in the pixels of the sensor array. After the lapse of a B signal accumulation time tbon12, the B LED are turned off by the signal ΦB. Then the R LED are turned on by the signal ΦR and the start pulse SP is entered again, whereby the B signals already accumulated in the pixels of the sensor array are simultaneously transferred to analog memories on the sensor array and then are output to the exterior pixel by pixel.
In this state the image sensor unit 200 has been moved to a next reading line, and similar operation are repeated for obtaining R, G and B signals. The entire color image is read by repeating the above-explained sequence by moving the image sensor unit 200 line by line in the subscanning direction.
In FIG. 36, the turn-on time tron12, tgon12, tbon12 of the R, G, B LED and the output periods tr12, tg12, tb12 of the R, G, B sensors are selected as tron12=tgon12=tbon12=tr12=tg12=tb12. This is achieved by adjusting the forward currents of the R, G, B LED of the color image sensor units in such a manner that predetermined sensor output levels are obtained for the R, G and B signals for an LED turn-on time same as the sensor output time for outputting the signals of all pixels.
Then, in the black-and-white original reading mode, the image sensor drive circuit 101 supplies the light source-switched image sensor unit 200 with the control signals ΦR, ΦG, ΦB, SP and CLK as shown in FIG. 37, wherein the turn-on time tron13, tgon13, tbon13 of the R, G, B LED and the black-and-white output period tw13 are selected as tron13=tgon13=tbon13=tw13 and tron13≠tron12. In this mode, the turn-on duty ratio of the LED is different from that in the color image reading because the R, G and B LED are simultaneously turned on for reading each line. Also in case of color image reading, the illuminating light intensity is so adjusted as to provide a predetermined sensor output when only one of the R, G and B LED is turned on. Because of these facts, the forward currents and the turn-on times of the R, G, B LEDs in reading the black-and-white original have to be made different from those in the color image reading mode.
The reading of the black-and-white original is executed in the following manner with the control signals shown in FIG. 37. At first all the LEDs of R, G and B colors are simultaneously turned on by the signals ΦR, ΦG, ΦB, and the sensor array is activated by the start pulse SP and the clock pulse CLK, whereby W signals corresponding to the black-and-white image are accumulated in the pixels of the sensor array. After the image reading of a line of the original, the image sensor unit 200 is moved to a next reading line and the start pulse SP is entered again, whereby the W signals already accumulated in the pixels of the sensor array are simultaneously transferred to the analog memories of the sensor array and then are output to the exterior in succession.
In this state, all the LEDs of R, G and B colors are turned on, whereby W signals of the next reading line are accumulated in the pixels of the sensor array. After the image reading of this line of the original, the image sensor unit 200 is moved to a further next reading line and the start pulse SP is entered again, whereby the W signal already accumulated in the pixels of the sensor array are simultaneously transferred to the analog memories of the sensor array and then are output to the exterior in succession. The black-and-white reading of the entire original is executed by repeating the above-explained sequence, with successive movement of the image sensor unit 200 by a line in the sub scanning direction.
As explained in the foregoing, such conventional light source-switched color image sensor can not only read the color image by illuminating each reading line of the original by turning on the R, G and B LEDs in succession and obtaining the output of the line sensor, but also read the black-and-white image by simultaneously turning on such R, G and B LEDs and obtaining the output of the line sensor.
However, in case of the black-and-white image reading, since the R, G and B LEDs are simultaneously and continuously turned on, the system may become unreliable if these LEDs are turned on under the same conditions as those in the color image reading. For this reason, the reliability of the system is maintained for example by reducing the currents supplied to the LEDs at the black-and-white image reading in comparison with those in case of the color image reading, but such operation complicates the LED driving circuit and the signal processing circuit, thereby elevating the costs thereof.